Laser diode driver with wave-shape control

ABSTRACT

An optical disk drive system associated with a laser diode is described. The optical disk drive system comprises a current generator for receiving input signals; a current switch coupled to receive timing signals; a current driver coupled to receive output signals from the current switch and the current generator, the current driver further comprising a driver with wave shape control selected from the group consisting of a laser diode read driver and a laser diode write driver, wherein the driver with shape control is operative for transmitting at least one output signal that is a scaled version of at least one of the output signals received from the current generator, wherein the current driver is operative for transmitting at least one output signal driving the laser diode.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a Divisional of prior Application Ser. No.13/662,712, filed Oct. 29, 2012, now U.S. Pat. No. 8,537,868, issuedSep. 17, 2013;

Which was a divisional of prior Application Ser. No. 12/758,160, filedApr. 12, 2010, now U.S. Pat. No. 8,325,583, issued Dec. 4, 2012;

The present application claims priority to jointly owned U.S.Provisional Application corresponding to application No. 61/186,298entitled “Laser Diode Write Driver with Wave-Shape Control.” Thisprovisional application was filed on Jun. 11, 2009. The presentapplication also claims priority to jointly owned U.S. ProvisionalApplication corresponding to application No. 61/186,299 entitled “LaserDiode Read Driver.” This provisional application was filed on Jun. 11,2009.

DESCRIPTION OF RELATED ART

With the evolution of electronic devices, there is a continual demandfor enhanced speed, capacity and efficiency in various areas includingelectronic data storage. Motivators for this evolution may be theincreasing interest in video (e.g., movies, family videos), audio (e.g.,songs, books), and images (e.g., pictures). Optical disk drives haveemerged as one viable solution for supplying removable high capacitystorage. When these drives include light sources, signals sent to thesesources should be properly processed so these sources emit theappropriate light for reading and writing data optically.

BRIEF DESCRIPTION OF THE DRAWINGS

The laser diode driver with wave shape control within the laser diodedriver signal processing system may be better understood with referenceto the following figures. The components within the figures are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the invention. Moreover, in the figures,like reference numerals designate corresponding parts or blocksthroughout the different views.

FIG. 1A, is a system drawing illustrating components within an opticaldisk drive.

FIG. 1B is a block diagram illustrating an enlarged view of theinnovative laser diode driver of FIG. 1A.

FIG. 2A is a circuit diagram illustrating a first implementation of thelaser diode write driver of FIG. 1B.

FIG. 2B is a circuit diagram of a current mirror with a beta-helper.

FIG. 2C is a circuit diagram of a Wilson current mirror.

FIG. 2D is a circuit diagram illustrating a second implementation of thelaser diode write driver of FIG. 1B.

FIGS. 3A-3B are circuit diagrams illustrating implementations of the ABdrivers of FIGS. 2A and 2D.

FIGS. 4A-4C are circuit diagrams illustrating a laser diode write driverwith wave shape control of FIG. 2A for altering rise time and fall time.

FIG. 5 is a circuit diagram illustrating a laser diode read driver ofFIG. 2A showing damping circuitry.

FIG. 6A is a circuit diagram illustrating a laser diode write driverwith wave shape control of FIG. 2A for showing over-shoot amplitude andpulse width control circuitry.

FIG. 6B is a comparative graph of the voltage for a data signal and thevoltage for an overshoot signal as a function of time.

While the laser diode driver with wave shape control is susceptible tovarious modifications and alternative forms, specific embodiments havebeen shown by way of example in the drawings and subsequently aredescribed in detail. It should be understood, however, that thedescription herein of specific embodiments is not intended to limit thelaser diode driver with wave shape control to the particular formsdisclosed. In contrast, the intention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of thelaser diode driver with wave shape control as defined by this document.

DETAILED DESCRIPTION OF EMBODIMENTS

As used in the specification and the appended claim(s), the singularforms “a,” “an” and “the” include plural referents unless the contextclearly dictates otherwise. Similarly, “optional” or “optionally” meansthat the subsequently described event or circumstance may or may notoccur, and that the description includes instances where the event orcircumstance occurs and instances where it does not.

Turning now to FIG. 1A, is a system drawing illustrating componentswithin an optical disk drive 100. A controller 102 monitors the outputlight power level of a laser diode 115 using a Monitor PD 104, ormonitor photodiode, and an RF, or radio frequency, preamplifier 106.This controller can keep an expected power level by changing an inputcontrol current of a laser driver 110 through an APC, or auto powercontrolling, feedback loop, even if a light source 115 such as a laserdiode changes output power due to various conditions, such astemperature changes, etc.

Also, the controller 102 sets the enable signal for switching somecurrent channels of the laser driver 110, which arranges a data writingpulse. In the case of data reading, the controller 102 may only set a DCcurrent by disabling the switching channels and applying the designatedcurrent. In the case of data writing, the controller 102 applies someadjustment signals, or enable-switching signals, to arrange the writingpulse waveform as a combination of switching current pulses. The powerlevel can be changed as each switching channel has its own designatedcurrent. The controller 102 can arrange these designated currents basedon the Monitor PD 104 output with some detecting function in the RFpreamplifier 106. At the very least, this controller has two powercontrol levels, one for the read power and one for the write power.

As illustrated in this figure, the laser driver 110 sends a signal thatprompts an associated light source 115 (e.g., laser diode) to emitlight. The light source 115 may emit light at any of a number ofwavelengths (e.g., 400 nm, 650 nm, 780 nm). Light from this sourcecontacts an associated optical media 120, such as a compact disc (CD),blue ray device (Blu-ray), or digital versatile disk (DVD). Lightcontacting the optical media can either facilitate data storage or dataretrieval from the optical media 120.

FIG. 1B is an enlarged view of the innovative laser driver 110, whichmay be a laser diode driver (LDD). The LDD 110 is an integrated, fullyprogrammable, multi-function product that controls and drives laserdiodes (e.g., light source 115) within optical drives as described withreference to FIG. 1A. More specifically, the LDD 110 can apply thecurrent for reading, writing, and erasing removable high capacity disks(e.g., capacities greater than approximately 50 Gbytes/disk). The LDD110 also has low noise (e.g., noise of approximately 0.5 nA/rt-Hz), highspeed (e.g., 800 Mb/s) and high current (e.g., approximately 1 amp). Anynumbers included in this application are for illustrative purposes onlyand numerous alternative implementations may result from selectingdifferent quantitative values.

At a high level, the LDD 110 may include a current generator 150.Generally, the current generator 150 receives some input signals 153associated with several input channels, which have an associated inputcurrent. The current generator 150 works in tandem with current driver160 and scales the input currents by some gain factors. The current atthe output 195 is typically a summation of these scaled input currentsfrom the individual channels. Thus, the current generator 150 andcurrent driver 160 control the amount of current for each output 195.Besides receiving current signals from the current generator 150, thecurrent driver 160 also receives signals from the current switch 155.The current switch 155 and the timing generator 175, via the serialinterface, control which of the channels should be turned on or turnedoff. The timing generator 175 receives various channel enable inputs190. Though there are five channel enable inputs that are shown in FIG.1B, the LDD 110 may have any number of channel enable inputs, such astwo, six, or the like. The timing generator 175 determines from thechannel enable inputs 190 and serial interface control, whether a giveninput channel will be either turned on or turned off and transmitscorresponding signals to the current switch. The current switch 155processes these signals and then transmits signals to the current driver160 designating which channels are active. The current driver 160 is thelast current gain stage and drives the laser diodes directly. In otherwords, the output signals from the current driver 160 also serve asoutput signals for the LDD 110, which are used in driving the laserdiode, or the light source 115 (see FIG. 1A).

In addition to the above-mentioned devices, the LDD 110 includesadditional components. A serial interface (I/F) 170 has several inputs(e.g., serial data enable, serial data, serial clock) that may be usedfor programming the gain, enabling channels, and turning on the LDD. TheLDD 110 also includes a high frequency modulator (HFM) 180 andvoltage/temperature monitor (V/Temp Monitor) 185. The HFM 180 modulatesthe output current for reducing mode-hopping noise of the laser diodes.The voltage/temperature monitor 185 monitors the laser diode voltagedrop and on-chip temperature. One skilled in the art will appreciatethat numerous alternative implementations may result from removing anyor several of the blocks within the LDD 110.

Though not illustrated, an integrated circuit for the LDD 110 generallyhas four switching, or write channels and one static, or read channelfor each output. Each driver can be programmed independently fromseveral milliamps to hundreds of milliamps. The current driver 160includes a Laser Diode Write Driver (LDWD) 165 for each output thatallows each switching channel to be programmed independently and hasvery fast switching times, low power, and good accuracy. The currentdriver 160 also includes a Laser Diode Read Driver (LDRD) 167 whichproduces a static current. The final output current is a summation ofeach individual switching channel from the LDWD and the static channelfrom the LDRD. The combination of the output currents from thesechannels are used to write data to the media

Either the LDWD 165 or the LDRD 167 can optionally include wave-shapecontrol circuitry. With this circuitry, each channel's wave-shape can beindependently controlled which includes overshoot, rise-time, andfall-time as further explained below. Altering the wave-shape canimprove the effectiveness in writing data to the optical media 120 (seeFIG. 1A) in the case of the LDWD 165.

FIG. 2A is one implementation of the LDWD 165 that shows a simplifiedcircuit diagram 200 of a single channel driver. Though optionally shownas a single channel driver, other implementations of the LDWD 165 mayinclude 2, 3, 5, or some other suitable number of channels. A currentsource 205 represents an input current, I_(input), which is a scaledversion of an output current, I_(out) based on a gain factor, K. As aresult, the input current I_(input)=I_(out)/K. As mentioned above, thisinput current may result from output current signals emitted from thecurrent generator 150 as shown in FIG. 1B.

The circuit diagram 200 includes several components that form a feedbackloop 207. The feedback loop 207 includes a transistor 210 that is shownas an n-type metal oxide semiconductor (MOS) transistor. Though shown asa MOS transistor, an alternative implementation may result from usingother transistor types such as a bipolar transistor. The size and othercharacteristics of the transistor 210 determine the current range overwhich the loop will function properly, the amount of headroom for theinput current source 205 and current mirror 250, and the accuracy of theloop. The feedback loop 207 also includes a resistor 215 coupled to alow voltage supply, or ground. The feedback loop 207 also includes twoAB drivers 230, 235 coupled in series. These series-connected AB driverscan be characterized by unity gain with a very high input impedance anda very low output impedance. The output of the AB driver 235 connects toa base of a transistor 240, which is connected in series with a resistor245. The size and other characteristics of the transistor 240 may bescaled to an output transistor 220, such that this output transistor isK times larger than the transistor 240 where K is a gain factor. Theresistor 245 may also be scaled to the output resistor 225, such thatits resistance may be the product of the resistance of the outputresistor 225 and the gain factor, K. Finally, the feedback loop 207 canalso include a capacitor 247 that sets a dominant pole within this loopfor stability.

The transistor 210 and the feed back loop work in concert. As an inputcurrent I_(input) enters this loop, the transistor's 210 gate will bedriven high until this transistor starts conducting current through theresistor 215, which is tied to the low supply voltage or ground. Avoltage develops across the resistor 215, which enters the AB driver230. The output voltage from AB driver 230 enters the AB driver 235; theresulting output voltage from the AB driver 235 correspondingly drivesthe base of the transistor 240. The voltage at the base of thetransistor 240 increases and it starts conducting. The feedback loop 207eventually settles at a point where all of the input current, orI_(out)/K, conducts through transistor 240 and the resistor 245 to theground.

As this feedback loop 207 reaches steady state, a voltage developsacross the resistor 215 that is equivalent to the voltage at the base ofthe transistor 240. This voltage is equal to the input current times theresistance value of 245 plus the base-emitter voltage of 240. At thatvoltage transistor 240 will conduct all of the input current. Thecurrent through resistor 215 (developed by the voltage across 215) goesthrough the transistor 210 into a current mirror 250. This currentmirror includes a transistor 252, resistor 254, transistor 256, and aresistor 258. For this current mirror, the current in transistor 252gets replicated into transistor 256.

An alternative implementation may include more complex current mirrorswith greater accuracy. For example, one alternative implementation mayinclude a beta-helper that helps reduce base current losses associatedwith the transistor 252 and the transistor 256 as shown in FIG. 2B. FIG.2C illustrates another alternative implementation which is a Wilsoncurrent mirror. The beta-helper implementation may be configured forunity gain or a higher gain that reduces the power.

The output current from the current mirror 250 enters a differentialpair. The differential pair includes the transistors 262 and 264. Thevoltages on the bases of these transistors determine which way thecurrent is steered. In other words, the base voltages determine whethercurrent goes through the transistor 262 to the ground or whether thecurrent goes through the transistor 264 and then the resistor 265 toground. If the current goes through transistor 264, it develops avoltage across the resistor 265. In one implementation, the resistanceof this resistor may have the same value as the resistor 215. Thevoltage across the resistor 265 will be the same as the voltage acrossthe resistor 215 because the current through the resistor 215 ismirrored to be the same through the resistor 265. In anotherimplementation, scaling the current from the current mirror 250 by afactor M and scaling the resistors such that 215 is M times larger than265 can also produce a voltage that is the same across these resistorswhile reducing power.

The circuit diagram 200 includes two current-mode ports 271, 272 thateither steers current into the resistor 265 or into ground. From thecurrent port 271, the devices that connect between this port and theground are as follows: transistor 273, resistor 275, and resistor 277.From the current port 272, the devices that connect between this portand the ground are as follows: transistor 274 and resistor 276, andresistor 277. With device 273 and device 274 set at a reference voltage,a voltage develops across an associated resistor depending on whetherport 271 of 272 is receiving current. For example, when current flowsthrough the transistor 274, a voltage may develop across the resistor276 and resistor 277. Similarly, when current flows through thetransistor 273, a voltage may develop across the resistor 275 andresistor 277. As the current switches between these transistors, theresistor 277 sets a common-mode voltage because it always has current init as the current is switched from port 271 to port 272 and back again.If device 273 is conducting current, resistor 275 develops a voltageacross it and the resistor 276 does not have a voltage across it so itwill be at the common-mode voltage; this means that the base of device264 is lower than the base of device 262 and the current conductsthrough device 264 into the resistor 265. The opposite is true when thecurrent is switched. The voltage across 275 or 276 is set such that thedifferential pair 262 and 264 switches completely. The common-modevoltage is set such that the device 264 does not saturate whenconducting current.

The voltage that develops across resistor 265 goes into a second pair ofseries connected AB drivers 282, 284 that are K times larger than the ABdrivers 230, 235; one skilled in the art will appreciate that each ABdriver may optionally be called a buffer, while two AB drivers mayoptionally be considered a buffering device. The characteristics ofthese AB drivers is the same as 230 and 235 which includes unity gain,high input impedance and low output impedance. Because AB drivers 282,284 are scaled versions, they will set a voltage on the base of thedevice 220 that is essentially the same as the voltage at the base ofthe device 240. Because device 220 is K times larger than device 240,the current through the device 220 will be K times larger; the outputcurrent I_(out), or driver output signal, is now a scaled version of theinput current I_(input) or input current signal.

This output current I_(out) conducts through an external laser diode(e.g. a laser diode that is associated with the light source 115), withits cathode receiving the output current I_(out); the correspondinganode for this laser diode will connect to another voltage supply. Thisoutput current I_(out) will conduct when the resistor 265 is set to thesame voltage as the resistor 215. When the current-mode inputs 271, 272are switched such that the device 262 conducts, the voltage across thedevice 265 will return to ground and the output driver 220 will shutoff. This write driver can be switched very quickly due to thecurrent-mode inputs 271, 272 and the differential pair that includesdevice 262 and device 264. The value of the resistor 265 can be chosenso the voltage quickly decays to ground when the current switches. TheAB drivers 282, 284 can be designed such that the voltage drop acrossthe device 265 is minimized. The voltage drop consists of a diode and asmall IR resulting in faster rise and fall times. The voltage at thebase of the output device 220 which is set by the voltage drop acrossresistor 265 and the design of the AB drivers 282, 284, determineswhether the output device is conducting or not conducting (on or off).

FIG. 2D is a second implementation of the LDWD 165 illustrating acircuit 290 that has its anode connected to I_(OUT) and its cathodeconnected to ground. Like the circuit 200, this circuit can drive alight source 115, such as a laser diode. One skilled in the art willappreciate that the circuit, though connected differently (essentially“flipped”), operates similar to the circuit 200.

FIG. 3A is a circuit diagram 300 that shows one implementation of the ABdrivers shown in FIG. 2A. Like FIG. 2D, FIG. 3B is a “flipped” versionof FIG. 3A, which operates like FIG. 3A. Returning to this figure, theAB driver 235 includes four transistors 310-316; an alternativeimplementation of this driver may include a different number oftransistors or transistors of a different type. Resistor 320 biases theAB driver 235. Device 316 receives an input voltage and produces a levelshifted output voltage at device 314 emitter which is received by thetransistor 240 (see FIG. 2A). The AB driver 284 includes the transistors330-336 and can be scaled versions of the transistors 310-316; this ABdriver can also produce a voltage to drive the output device 220. Likethe resistor 320, the resistor 340 can bias the AB driver 284; theresistor 340 is K times smaller than the resistor 320, while thetransistors 330-336 are K times larger than the transistors 320-326.

The AB driver 230 also includes four transistors 350-356 biased bycurrent source 358; together these transistors receive an input voltageat device 352 base (which is the voltage across resistor 215) and levelshift that voltage up a diode and output it at device 350 emitter.Similarly, the AB driver 282 also includes four transistors 360-366 anda bias current source 368. Device 360 receives an input voltage and thatvoltage is level shifted and output at device 362 emitter. Thetransistors 360-366 and current source 368 may be scaled versions thatare K times larger that the transistors 350-356 and current source 358.

Using components within the circuit 300, designers can make selectionsthat improve the power and the speed of the LDWD 165. Optimizing somecurrent sources (e.g., current source 358, current source 368) orresistors (e.g., resistor 320, resistor 340) within the circuit 300 candramatically improve the power or the speed. For example, increasingbias currents will make the AB drivers have a lower output impedance sothey can drive the output device faster, but this increases power.Decreasing the current will typically slow down the switching of theoutput device. In addition, the AB drivers 230, 235, 282, and 284 may beconfigured such that the input into the AB driver 230 gets level shiftedup one diode to its output and the input from the AB driver 235 getslevel shifted down one diode to its output; thus the voltage drop acrossthe resistor 215 is essentially equal to a diode and IR. This is thesame for AB drivers 282 and 284. This impacts the speed and performanceof this LDWD because the voltage drop that is across resistor 215 andalso 265 is minimized and there is always parasitic capacitancesassociated with interconnect, etc and so the lower the voltage swingtypically the faster the switching. In addition, the gain factor K canalso be chosen for accuracy, speed, and power optimization. Somepotential values for this gain factor may be 20, 40, or the like.

The LDWD 165 may also include wave shape control, which may change therise-time, fall-time, or overshoot of the output current waveform. FIG.4A is a circuit diagram 400 for the LDWD 165 with wave shape control.Though similar to the circuit diagram 300 and similar devices arenumbered the same, the circuit diagram 400 impacts wave shape control byincluding two devices. More specifically, this circuit diagram includesa rise-time variation device 410, which includes a switch, ortransistor, 411 and a capacitor 413 connected to the bases of transistor360 and the transistor 362. As current travels through the transistor264 to the resistor 265, the AB driver 282 tracks the voltage across265. The voltage on the resistor 265 develops quickly because of lowcapacitive loading on this node. This creates a fast rise-time thattransfers through the AB driver 282 and the AB driver 284 to the outputtransistor 220. When the transistor 411's gate is at ground, it is offand there is high impedance between the drain and the source, such thatthe capacitor 413 has little effect on the resistor 265.

In contrast, changing the gate of transistor 411 to a voltage ofapproximately VCC turns on this transistor and there is low impedancefrom drain to source. Now, capacitor 413 is in parallel with theresistor 265. Thus, current from the transistor 264 charges both thiscapacitor and this resistor, which means that it takes longer for thecurrent from the transistor 264 to reach its steady-state value. Thevoltage at the base of the output transistor 220 follows the input tothe AB driver 282, which is essentially the voltage across the resistor265. Since this voltage is now slower and the output transistor follows,the output current rise-time is slower. Therefore, including therise-time variation device 410 can alter the rise-time of an outputsignal from the circuit diagram 400 for the LDWD 165. In anotherimplementation, selecting certain device characteristics can create adesired output rise time. For example, one may select a certain size forthe transistor 411 or a certain capacitance for the capacitor 413.Adding another rise-time variation device 415 in parallel with device410 can make programmable rise times as shown in FIG. 4B. Though thedevices within the rise-time variation device 415 are not shown, theymay be either active or passive. In one implementation, they may includea MOS field effect transistor and a capacitor of a different value thanthe capacitor 411 and the transistor 413. Adding these devices canfurther slow the rise-time, which may further reduce overshoot andringing, thus controlling the waveshape.

FIG. 4C is a circuit diagram 450 of an alternative implementation forthe LDWD 165 with wave-shape control circuitry where the rise-time canbe controlled. This circuit diagram includes a resistor 453 positionedbetween the AB driver 282 and the AB driver 284. Adding a resistor 455in the control loop that is K times larger than the resistor 453 cancompensate for any voltage drop across the resistor 453. The resistor453 increases the output impedance of the AB driver 284 and helpsisolate it from ringing; this ringing may be associated with either oneor both of the voltage supplies due to inductance. The ringing can alsobe associated with driving a laser diode that has inductance associatedwith its package. If a rise-time variation device 460 connects to thebases of the transistor 334 and the transistor 336 within the AB driver284, the AB driver 282 charges the capacitor 462 through the resistor453 which also slows down the rise-time of the voltage that drives ABdriver 284 so long as the gate of the transistor 464 is connected tovoltage such that the device is on. When the transistor 464 is turnedoff, the capacitor 462 has very little effect on the rise-time. As withthe other solution described with reference to FIG. 4B, several moredevices can be added in parallel to make programmable rise-times byusing different values of capacitors.

Returning to FIG. 4A, the circuit diagram 400 also includes a fall-timevariation device 420 connected to the bases of the transistor 330 andthe transistor 332 within the AB driver 284. The rise-time variationdevice 410, which decreases the rise time, generally does not have alarge effect on the fall-time. The reason is because the resistor 340 isthe most impactful on the fall-time along with the node that connectsthis resistor to the bases of the transistor 330 and the transistor 332.When the input to AB driver 284 decreases, the transistor 332 pulls thebase of the transistor 220 down as its base is pulled down by resistor340. When the transistor 332 has a smaller voltage V_(BE) than thetransistor 220, the transistor 332 can effectively turn off thetransistor 220. When there is little capacitance on the base of thetransistor 332, then the resistor 340 can quickly pull this base to alow supply voltage or ground. If there is capacitance on that node, thenit takes longer to pull the node to ground and the transistor 220 takeslonger to shut off. The fall-time variation device 420 may include aswitch, or transistor 422, and a capacitor 424. The transistor 422 is aswitch that is on or off and either adds the capacitor 424 to thecircuit or acts as a high impedance that has little effect on thecircuit. As described with reference to FIG. 4B, several more devices(e.g., mosfets and capacitors) can be added in parallel to accommodatedifferent fall-times. The fall-time variation device 420 has littleeffect on the rise-time because when the AB driver is pulled high, thetransistors 334 and 336 provide the current to pull-up on the base ofthe output transistor.

As mentioned above, rise-time and fall-time variation devices can shapethe output signals emitted from a driver. While the waveshape controldescribed is applicable to a write driver, the read driver can alsoaffect the waveshape. Returning to FIG. 1B, the LDD 110 may also includea laser diode read driver (LDRD) 167 that has wave shape controlcircuitry and can consequently impact an output signal 195. FIG. 5 is acircuit diagram 500 for the LDRD 167 that includes a wave shape controldevice 580.

FIG. 5 is a simplified circuit diagram 500 for one implementation of theLDRD 167. For this implementation, the circuit's cathode connects to thepin I_(OUT) and the circuit's anode connects to a positive voltagesupply. When there is a desired output current, the LDRD 167 can bedesigned to produce this desired current as illustrated in the circuitdiagram 500. For example, the desired output current may be I_(OUT) andthe circuit diagram 500 may have a gain K associated with it. To producethis output current, the circuit diagram 500 receives an input referencecurrent I_(OUT)/K shown as current source 505, where K is the gainfactor; this current may be from a current source, such as a previousstage in the current generator block 150. As this reference currententers this circuit, the current reaches ground by traveling throughtransistor 510 and resistor 515. While the transistor 510 is shown as annpn bipolar junction transistor, other implementations may result fromusing different transistor topologies. The transistor 510 is also adiode-connected transistor. The size of transistor 510 can be scaled toan output transistor 520 by the inverse of the gain factor, or 1/K,(e.g., area of transistor 510 may equal area of transistor 520*1/K). Ascurrent flows from current source 505 through transistor 510, it reachesresistor 515 and then encounters ground. As transistor 510 is scaled totransistor 520, resistor 515 can be scaled to the output resistor 525;for example, the value of resistor 515 can be the product of resistor525 and the gain K. Matching the device 510 with the device 520 and thedevice 515 with the device 525 can improve the accuracy of the outputcurrent I_(OUT) in relation to the input current I_(OUT)/K.

The input reference current I_(OUT)/K 505 sets a reference voltage atthe V_(N) terminal 532 of the transconductor 530. The transconductor 530has two input terminals and produces a current signal reflective ofdifferences between signals received on its input terminals. Asmentioned above, the transconductor 530 includes a V_(N) terminal 532and V_(P) terminal 534 where V_(N) is the voltage applied to theterminal 532 and V_(P) is a voltage applied to the terminal 534. Thevalues for these voltages may be the sum of (I_(OUT)/K)*Resistor 515 andthe voltage of the diode connected transistor 510 or the like. Thetransconductor 530 produces an output current signal on terminal 536that reflects a difference of the signals received on the terminal 532and the terminal 534. The output current signal has an associated outputcurrent I where I=GM*(VP−VN). In this formula, GM is thetransconductance of the transconductor 530, which may have a value 20 uSor the like.

As the output current signal emerges from the transconductor 530, itdrives the capacitor 540. The size of this capacitor for this particularapplication is around 15 pF. The capacitor 540 can filter noise presentin the output current signal that may be associated with a previousstage in the laser diode driver 110. In other words, noise in the outputsignals from the current generator 150 (see FIG. 1B) may appear as noiseon the input terminal 532, which would appear as noise in the outputcurrent signal on the terminal 536. It is this noise in the outputcurrent signal that capacitor 540 can filter. The size for thistransistor may be selected based on design parameters to get a desiredamount of filtering.

The output current signal from the transconductor 530 also drives ametal oxide semiconductor (MOS) transistor 550. While shown as a MOStransistor, one skilled in the art will appreciate that the specifictype of transistors within the LDRD 167 and the circuit 500 may varydepending on design objectives. This output current signal drives thegates of the transistor 550 to a voltage such that the voltage V_(P)equals the voltage V_(N) by outputting a current into the transistor 560and the resistor 565, which goes to a low voltage supply. The size ofthe transistor 560 can scale to the transistor 510 or the transistor520, if desired. Similarly, the resistance of the resistor 565 can scaleto the resistor 515 or the resistor 525, if desired. In addition, thetransistor 560 and the resistor 565 form a current mirror 570 thatconnects to the base of output transistor 520, the terminal 534 of thetransconductor 530, the drain of the transistor 550, and the low voltagesupply or ground.

The LDRD 167 illustrated by the circuit diagram 500 has an effectiveoperation. As briefly mentioned above, this circuit diagram includes ahigh voltage supply V_(CC), which may have a voltage of 5V associatedwith it. Current source 505, capacitor 540, and transistor 550 allconnect to this voltage supply. In contrast, resistors 515, 525, and 565all connect to the low voltage supply, or ground. Due to the closed loopor the connection of the current mirror leg 570, the transconductor 530,and the transistor 550, the voltage at the base of the transistor 560and the base of the transistor 520 will be the same as the voltage onthe base of the transistor 510. In other words, the voltage Vn at thebase of transistor 510 terminal 532 equals the voltage V_(P) on terminal534 as explained above, which is applied the bases of the transistor 520and the transistor 560. Because transistor 520 and resistor 525 arescaled to the transistor 510 and the resistor 515, the output currentI_(out) or current emerging from the LDRD 167 and the circuit diagram500 will be a scaled replica of the input current by the gain factor K.

For the LDRD 167, the circuit diagram 500 may also include a waveshapecontrol device 580 that may include one or more either active or passivedevices. For this implementation, the device 580 includes a resistor 583and a capacitor 585. The waveshape control device 580 connects to thebase of the transistor 560 and the output terminal. Four switchingchannels may also be connected to this output node. Together theresistor 583 and the capacitor 585 can dampen the swing on the outputsignal, which reduces the overshoot and undershoot of the diode lasercurrent. If the switching channels are increasing in current, thevoltage on the output node I_(OUT) would decrease, which tends to shutoff or decrease the output current from transistor 520. The overalleffect would temporarily decrease the output current while a switchingchannel is turning on having a dampening effect on the output waveform.The opposite occurs as current is decreased in the laser diode. Incontrast, when the switching channels are decreasing in current, thevoltage on the output node I_(OUT) would increase, which tends to turnon the output transistor 520 harder, which increases the output current.This reduces the undershoot as the current from a switching channel isbeing turned off.

Returning to the LDWD 165, there is another implementation of waveshapecontrol circuitry that can be used for controlling the overshoot. FIG.6A is a circuit diagram 600 that includes a device 610 for controllingovershoot. In this implementation, the waveform overshoot can becontrolled explicitly by adding a pulsing current source that pulsesinto the resistor 265 of the write driver channels for a specifiedduration and amplitude; each channel being independent. This increasesthe overshoot of a specific channel if desired. The device 610 includesswitches, or transistors, 611-614 and resistors 616-618. In this device610 like the data path, there is a differential pair, which includestransistors 612-613 that can either steer current into the resistor 265or to ground. The tail current is I_(OUT)/M, where M is a scale factor.When the current is pulsed into the resistor 265, it increases thevoltage; this increases the voltage on the base of the output transistor220, which increases the output current. Turning to FIG. 6B, this figureis a comparative graph of the voltage for a data signal 620 and thevoltage for an overshoot signal 630 as a function of time. As can beseen from the figure, the overshoot data is only on for a short periodof the time the data signal is on, or switching time as shown.

With the LDWD shown in circuit diagram 600, the channel driver can beconfigured independently of the others, and its switching isindependent. The driver has a very large dynamic range and the accuracydepends on the gain factor K and device matching. When properly scaled,the driver has very low power and provides very fast switching of thedata. In addition, adjusting one of the wave-shape controls has verylittle impact on the other controls. The wave-shapes can be modified inseveral ways including rise-time, fall-time and overshoot to make awaveform that gives the best performance. Also, each of the controls iseasily programmable with a minimal amount of additional circuitry.Finally, this wave shape control can be done in either the LDWD 165 orthe LDRD 167.

While various embodiments of the laser diode driver with wave-shapecontrol have been described, it may be apparent to those of ordinaryskill in the art that many more embodiments and implementations arepossible that are within the scope of this system. Although certainaspects of the laser diode driver with wave-shape control may bedescribed in relation to specific techniques or structures, theteachings and principles of the present system are not limited solely tosuch examples. All such modifications are intended to be included withinthe scope of this disclosure and the present laser diode driver withwave-shape control and protected by the following claim(s).

What is claimed is:
 1. A laser diode read driver circuitry comprising:A. an input current source coupled between Vcc, a first transistor, afirst resistor and ground; B. transconductor circuitry having one inputconnected between the input current source and the first transistor,having another input, and an output; C. a second transistor, a thirdtransistor, and a second resistor coupled between Vcc and ground, thesecond transistor having an input coupled to the output of thetransconductor circuitry, the third transistor having an input, and theanother input of the transconductor circuitry being coupled to betweenthe second transistor and the third transistor; D. a fourth transistorand a third resistor coupled between an output and ground, the fourthtransistor having an input connected to the input of the thirdtransistor and to between the second transistor and the thirdtransistor, the fourth transistor having an area; and E. wave shapingcircuitry coupled between the inputs of the third and fourth transistorsand the output.
 2. The circuitry of claim 1 in which the firsttransistor has a first area that is scaled to an area of the fourthtransistor by the inverse of a gain factor K.
 3. The circuitry of claim1 in which the wave shaping circuitry includes a capacitor and a fourthresistor coupled in series.